The first hundred attoseconds of the electron dynamics during strong field tunneling ionization are investigated. We quantify theoretically how the electron's classical trajectories in the continuum emerge from the tunneling process and test the results with those achieved in parallel from attoclock measurements. An especially high sensitivity on the tunneling barrier is accomplished here by comparing the momentum distributions of two atomic species of slightly deviating atomic potentials (argon and krypton) being ionized under absolutely identical conditions with near-infrared laser pulses (1300 nm). The agreement between experiment and theory provides clear evidence for a nonzero tunneling time delay and a nonvanishing longitudinal momentum of the electron at the "tunnel exit."

The evolution of a V-type three-level system is studied, whose two resonances are coherently excited and coupled by two ultrashort laser pump and probe pulses, separated by a varying time delay. We relate the quantum dynamics of the excited multilevel system to the absorption spectrum of the transmitted probe pulse. In particular, by analyzing the quantum evolution of the system, we interpret how atomic phases are differently encoded in the time-delay-dependent spectral absorption profiles when the pump pulse either precedes or follows the probe pulse. This scheme is experimentally applied to atomic Rb, whose fine-structure-split 5s  (2)S{1/2}→5p(2)P{1/2} and 5s(2)S_{1/2}→5p(2)P{3/2} transitions are driven by the combined action of a pump pulse of variable intensity and a delayed probe pulse. The provided understanding of the relationship between quantum phases and absorption spectra represents an important step towards full time-dependent phase reconstruction (quantum holography) of bound-state wave packets in strong-field light-matter interactions with atoms, molecules, and solids.

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We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2p-3d boundbound transitions between the spin-orbit multiplets 3 P0,1,2 and 3 D1,2,3 of the transiently produced doubly charged Ne 2+ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4 ± 0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the AC Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron-laser radiation when the phase-locked pump and probe pulses precisely overlap in time. PACS numbers: .In interaction with matter the oscillating electric field of a laser not only induces transitions between bound electronic states but also affects the states and transitions themselves. It splits [1], shifts [2, 3] and modifies the width [4, 5] and the shape [6-8] of spectral transition lines depending on the amount of detuning out of resonance with the laser frequency and the field strength. Only for sufficiently high field strengths, at which more than one photon can interact with the quantum system on its intrinsic time and energy scale, these phenomena are accessible. Modern ultrafast lasers are effective driver and control tools for nonlinear effects at visible frequencies and have become the "working horses" for nonlinear coherent spectroscopies [9], in time domain and frequency domain, including the quantum control of boundbound electronic transitions (see, e.g., [10] and references therein).Since the advent of short-wavelength free-electron lasers (FELs) [11,12] the field of nonlinear spectroscopy is being extended into the extreme-ultraviolet (XUV) and x-ray spectral ranges [13][14][15][16][17][18][19][20][21][22][23][24]. One advantage of employing x-rays is the ability to access bound-bound electronic transitions associated with the spatially localized innerelectronic shell and the potential to probe site-specific spectroscopic information of a sample. Since experimental studies on the impact of XUV/x-ray nonlinear effects on inner-shell-excited resonances are often ham-pered by the extremely short Auger decay times, yet, such research is rare. Nonetheless, first x-ray nonlinear line-shape modifications of inner-shell transitions have been studied experimentally [16] by employing Augerelectron spectroscopy. By contrast, we here address the valence electrons of the doubly charged neon ion, Ne 2+ , and manipulate-in the absence of any competing ultrafast decay channel-the ground state to excited state transitions between spin-orbit multiplets with intense XUV-FEL radiation. Being sensitive to the atomic/ionic dipole response and associated spectral line-shape modifications, our work demonstrates a direct view on XUV nonlinear effects occurring i...